Vertical gate transistor and pixel structure comprising such a transistor

ABSTRACT

The present disclosure relates to a photodiode comprising: a P-conductivity type substrate region, an electric charge collecting region for collecting electric charges appearing when a rear face of the substrate region receives light, the collecting region comprising an N-conductivity type region formed deep in the substrate region, an N-conductivity type read region formed in the substrate region, and an isolated transfer gate, formed in the substrate region in a deep isolating trench extending opposite a lateral face of the N-conductivity type region, next to the read region, and arranged for receiving a gate voltage to transfer electric charges stored in the collecting region toward the read region.

BACKGROUND

1. Technical Field

The present disclosure relates to sensors, in particular CMOS technologysensors, and more particularly to a structure of light-sensitive pixelof BSI type (Back-Side Illumination).

2. Description of the Related Art

Such image sensors comprise a network of pixels formed in asemiconductor substrate. Each pixel comprises a photodiode coupled to aread node by a transfer transistor. The electric charges accumulated bythe photodiode during an integration time can be transferred to a readnode by the transfer transistor. The voltage of the charges accumulatedat the read node is read by a read circuit generally comprising a sourcefollower transistor, the gate of which is coupled to the read node. Theread circuit also comprises a reset transistor, also coupled to the readnode, to reset to zero the voltage at the read node after each read. Toreduce the number of components, one well-known method involves sharingthe read circuit between several photodiodes.

The sensitivity and thus the quality of the image sensor are inparticular related to the capacity of each photodiode to store electriccharges. In particular, if during the integration time, severalphotodiodes can no longer store electric charges, and thus reach asaturation state, the quality of the image supplied by the image sensorwill be affected.

To reduce the size of pixels and increase their sensitivity, onewell-known method also involves forming the photodiodes in the thicknessof the substrate so as to be capable of capturing the light via the rearface of the substrate (BSI technology), whereas the transfertransistors, the read circuits and the electrical links between thepixels and a circuit for managing the image sensor are formed on thefront face of the substrate. In this way, the entire rear face of thesubstrate can be used to capture the light.

FIGS. 1 and 2 are cross-sections and top views representing a pair ofpixels 1, 2 of a BSI-type image sensor. FIG. 3 schematically representsa circuit formed by the pair of pixels in FIGS. 1 and 2. The pixels 1, 2are formed in a silicon substrate 3, in which pixel regions aredelimited by perpendicular deep isolating trenches 6. Each pixelcomprises a photodiode PD11, PD12 formed in a slightly dopedP-conductivity type region 4, 5 of the substrate 3, extending from thesurface of the substrate 3 over most of the thickness of the latter.Each photodiode PD11, PD12 comprises a slightly doped N-conductivitytype region 13, 23, formed in the region 4, 5. The region 13, 23 extendsover most of the thickness of the region 4, 5. Charges resulting fromthe light reaching the region 4, 5 via the rear face of the substrate 3accumulate in the region 13, 23. These charges are transferred during atransfer phase toward a read node 10, 20, by a transfer transistor T11,T12 comprising a gate stack 11, 21 formed on the front face of thesubstrate 3 above the area 13, 23. The read node 10, 20 comprises aheavily doped N-type read region 14, 24 forming a conduction region ofthe transfer transistor T11, T12. During the transfer phase, chargesleave the region 13, 23 by the side and top, toward the read node 10,20, passing through a channel produced by applying a transfer voltage tothe terminal 12, 22 of the gate stack 11, 21 of the transfer transistorT11, T12.

To reduce the number of components per pixel, the pair of pixels 1, 2comprises a single reset transistor T13 and a single source followertransistor T14, the read nodes 10 and 20 of the two pixels 1, 2 beinginterconnected by a link 7. The transistor T13 comprises a gate stack31, a conduction region corresponding to the read region 14, shared withthe transfer transistor T11 and a conduction region receiving a voltageVRS. The transistor T14 comprises a gate stack 41 connected to the readnodes 10, 20 and a conduction region receiving a supply voltage VSF. Thearea of the substrate 3 beneath the gate stack 41 is separated from theread region 24 of the read node 20 by a shallow trench STI. The quantityof charges accumulated by each of the photodiodes PD11, PD12 can beseparately measured by means of the transistor T14, by successivelycontrolling the two transfer transistors T11, T12, to transfer thecharges accumulated by each photodiode at the read node 10, 20. Betweeneach charge transfer, the reset transistor T13 can be controlled toreset the voltage of the read node 10, 20.

It is desirable to increase the sensitivity of the photodiodes of such apixel structure, without increasing the surface area of substrateoccupied by the pixel.

BRIEF SUMMARY

Some embodiments relate to a photodiode including a P-conductivity typesubstrate region, an electric charge collecting region for collectingelectric charges appearing when a rear face of the substrate regionreceives light, the collecting region comprising an N-conductivity typeregion formed deep in the substrate region, an N-conductivity type readregion, formed in the substrate region, and an isolated transfer gate,disposed in the substrate region next to the N-conductivity type regionand to the read region, and arranged for receiving a gate voltage totransfer electric charges stored in the collecting region toward theread region. According to one embodiment, the transfer gate is formed ina deep isolating trench extending opposite a lateral face of theN-conductivity type region.

According to one embodiment, the photodiode comprises a conductivecolumn linked to the transfer gate and passing through an isolatinglayer to reach a front face of a substrate comprising the substrateregion and the isolating layer.

According to one embodiment, the transfer gate has a lateral surfacecorresponding to and disposed opposite a right or left half of a lateralface of the N-conductivity type region.

According to one embodiment, the transfer gate extends in the substrateregion over a depth of 1.5 to 3.5 μm, and over a width of 0.1 to 0.3 μm.

According to one embodiment, the transfer gate is isolated from thesubstrate region by an isolating layer the thickness of which is between10 and 20 nm.

According to one embodiment, the collecting region has a symmetricalstructure comprising two N-conductivity type regions and two transfergates each disposed between one of the two N-conductivity type regionsand the read region, to transfer electric charges stored in thecollecting region toward the read region.

According to one embodiment, the collecting region has a symmetricalstructure in which the N-conductivity type region has a U-shaped sectionin a plane parallel to the front face of the substrate region.

According to one embodiment, the gate has a U-shaped section in a planeparallel to the front face of the substrate region and is disposedbetween the branches of the U shape of the N-conductivity type regionand around the read region.

Some embodiments also relate to an image sensor pixel comprising thephotodiode previously defined.

According to one embodiment, the read region forms a conduction regionof a reset transistor provided to reset a voltage of the read region.

According to one embodiment, the pixel comprises a source followertransistor comprising a gate terminal linked to the read region, thesource follower transistor being formed above the N-conductivity typeregion, an isolating layer being arranged between the source followertransistor and the N-conductivity type region.

Some embodiments also relate to an image sensor comprising severalpixels like the one previously defined, the pixels being isolated fromeach other by deep isolating trenches or embedded vertical gates.

Some embodiments also relate to an electronic device comprising theimage sensor previously defined.

Some embodiments also relate to a method for manufacturing an imagesensor comprising the formation of pixels, the formation of each pixelcomprising steps of: forming in a P-conductivity type substrate regionan electric charge collecting region for collecting electric chargesappearing when a rear face of the substrate region receives light, thecollecting region comprising an N-conductivity type region formed in thesubstrate region, forming in the substrate region an N-conductivity typeread region, forming in the substrate region a deep isolating trenchextending opposite a lateral face of the N-conductivity type region andnext to the read region, and forming an isolated transfer gate in theisolating trench, the transfer gate being arranged for receiving a gatevoltage to transfer electric charges stored in the collecting regiontoward the read region.

According to one embodiment, the substrate is of silicon-on-insulatortype.

According to one embodiment, the read region and a substrate bias regionare formed by epitaxy of a semi-conductive material in a hole passingthrough an isolating layer of the substrate to reach the substrateregion.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

These and other objects, characteristics and advantages of the presentdisclosure will be explained in greater detail in the followingdescription of specific embodiments given below in relation with, butnot limited to, the following figures, in which:

FIG. 1 described above is a cross-section of a pair of pixels, accordingto prior art,

FIG. 2 described above is a top view of the pair of pixels in FIG. 1,

FIG. 3 described above schematically represents an electric circuitformed by the pair of pixels in FIG. 1,

FIG. 4 schematically represents an electric circuit of a pixel,according to one embodiment,

FIG. 5 is a top view of a pixel forming the electric circuit in FIG. 4,according to one embodiment,

FIGS. 6A, 6B, 6C, 6D, 6E, 6F and 6G are cross-sections of the pixelrepresented in FIG. 5, according to planes AA′, BB′, CC′, DD′, EE′, FF′and GG′ indicated on FIG. 5,

FIG. 7 is a cross-section of the pixel according to a plane HH′ parallelto the surface of the substrate, indicated on FIGS. 6A and 6G,

FIGS. 8A, 8B, 8C, 8D, 8E and, 8F are cross-sections of a substrate,according to a plane II′ indicated on FIG. 5, showing steps ofmanufacturing the pixel in FIG. 5,

FIGS. 8G and 8H are cross-sections of the substrate in FIGS. 8A to 8F,according to the plane GG′ indicated on FIG. 5, showing steps ofmanufacturing the pixel in FIG. 5,

FIG. 9 is a top view of a pixel, according to another embodiment,

FIG. 10 is a cross-section of the pixel in FIG. 9, according to a planeparallel to the surface of the substrate,

FIG. 11 is a top view of a pixel, according to another embodiment,

FIG. 12 is a cross-section of the pixel in FIG. 11, according to a planeparallel to the surface of the substrate,

FIG. 13 is a top view of a pixel, according to another embodiment.

DETAILED DESCRIPTION

FIG. 4 represents a pixel structure according to one embodiment. Thepixel structure PXL comprises two photodiodes PD1, PD2, two transfertransistors T1, T2, a reset transistor T3 and a source followertransistor T4. The transistor T1 comprises a conduction terminalconnected to the photodiode PD1, and a conduction terminal connected toa common read node SN. The transistor T2 comprises a conduction terminalconnected to the photodiode PD2, and a conduction terminal connected tothe read node SN. Each of the transistors T1, T2 comprises a controlterminal receiving a same control signal VTG. The transistor T3comprises a conduction terminal connected to the read node SN, aconduction terminal receiving a supply voltage VRS and a controlterminal receiving a control signal RST. The transistor T4 comprises acontrol terminal connected to the read node SN, a conduction terminalreceiving a supply voltage VSF and a conduction terminal subjected to avoltage VX.

FIGS. 5, 6A, 6B, 6C, 6D, 6E, 6F and 6G represent the pixel structure PXLproduced in an SOI-type substrate (Silicon On Insulator), according toone embodiment. The substrate comprises a substrate region PPW in asemi-conductive material, covered on a front face with a thin layer madeof an isolating material IL1. The layer IL1 is covered with an isolatinglayer IL2 that is thicker than the layer IL1. The substrate region PPWcan be of slightly doped P-conductivity type. The pixel structure PXLcan be part of a sensor comprising pixels arranged in rows and incolumns, the rows of pixels and the columns of pixels being isolatedfrom each other by deep isolating trenches DTI formed in the substratefrom the front or rear face of the latter. The pixel PXL comprises twoslightly doped N-conductivity type wells PNW1, PNW2, implanted deep inthe substrate region PPW to form with the latter the photodiodes PD1,PD2. The wells PNW1, PNW2 extend deep into the substrate region PPW,over most of the depth of the trenches DTI beneath the layer IL1(between 75 and 95% of the depth of the trenches) and over most of thewidth (between 85 and 95% of the distance) between two trenches DTI. Thewells PNW1, PNW2 are relatively narrow, having a width lower than or inthe vicinity of one third of the distance between two trenches DTI.Thus, the N-conductivity type wells PNW1, PNW2 are crimped between twoP-conductivity type regions, thus forming narrow and deepparallelepipedal regions. The wells PNW1, PNW2 can have awidth-to-length ratio of 1/3.

Above an area between the wells PNW1, PNW2, the pixel structure PXLcomprises a substrate bias region BBP, a read region SNN, a gate stackRSG and a conduction region RSN of the reset transistor T3 (FIGS. 5, 6A,6B, 6C, 6F). The substrate bias region BBP and the read region SNN areeach formed by a semi-conductive layer filling a hole passing throughthe layers IL1, IL2 and reaching in the substrate region PPW a depthsubstantially corresponding to that of the high parts of the wells PNW1,PNW2. The substrate bias region BBP has a heavily doped P-conductivitytype and is linked to a substrate bias contact BB above the layer IL2.The read region SNN has an N-conductivity type which can be heavilydoped in its top part SNN1 (front face of the substrate) and less dopedin its lower part SNN2. The region SNN1 is linked above the layer IL2 tothe read node contact SN. The gate stack RSG is formed above aP-conductivity type semi-conductive channel region RSP, formed in thelayer IL2. The gate stack RSG is linked above the layer IL2 to a contactreceiving the control voltage RST. The heavily doped N-conductivity typeconduction region RSN is also formed in the layer IL2. The region RSN islinked above the layer IL2 to a contact receiving the voltage RSV. Theconduction regions SNN, BBP through the layers IL1, IL2 and theconduction regions RSP, RSN in the layer IL2 can be formed by epitaxy ofa semi-conductive material such as silicon.

In particular on FIGS. 5, 6B and 6E, the transistor T4 is formed in theisolating layer IL2, above one of the wells PNW1, PNW2, for example thewell PNW1. The transistor T4 comprises two N-conductivity typeconduction regions SFN1, SFN2, separated by a P-conductivity typechannel region SFP. A gate stack SFG is formed above the region SFP. Theregion SFN1 is linked above the layer IL2 to a contact subjected to avoltage VX. The region SFN2 is linked above the layer IL2 to a contactsubjected to the voltage VSF. On FIGS. 5 and 6B, the region SNN1 and thegate stack SFG are linked to the contact SN.

In particular on FIGS. 5, 6C, 6D, and 6G, the pixel structure PXL alsocomprises deep vertical gates TG1, TG2 of the transfer transistors T1,T2, formed between the wells PNW1, PNW2. The read region SNN is betweenparts of the gates TG1, TG2 located in the vicinity of the center of thepixel structure PXL. Each gate TG1, TG2 is formed in a deep trench thelateral walls and the bottom of which are covered with a thin layer ofisolating material TGI1, TGI2, and which is filled with a conductivematerial such as polycrystalline silicon, up to a level lower than theisolating layer IL1. The top part (front face of the substrate) of thetrench forming each of the gates TG1, TG2 is filled with an isolatingmaterial. Each gate TG1, TG2 comprises a column TGC1, TGC2 establishingan electric link between the conductive material forming the gates TG1,TG2 and the surface of the layer IL2 along one of the isolating trenchesDTI. The columns TGC1, TGC2 of the gates TG1, TG2 can be linked to eachother above the layer IL2 by a conductive link TG. It shall be notedthat the columns TGC1, TGC2 are not necessarily formed along a trenchDTI, but can be formed in another location above the trenches in whichthe gates TG1, TG2 are formed.

On FIG. 6G, each gate TG1, TG2 extends and is opposite substantially theentire height and the right or left half of the lateral face of one ofthe wells PNW1, PNW2, the wells PNW1, PNW2 themselves extending overmost (between 85 and 95%) of the distance between two trenches DTI andover most (between 75 and 95%) of the depth of the trenches DTI beneaththe layer IL1.

In the example in FIG. 7, the photodiode region formed in the substrateregion PPW has a symmetrical structure comprising two wells PNW1, PNW2and two gates TG1, TG2, spread over each side of a symmetry plane XX′,as well as a bias region BBP and a read region SNN passed through by thesymmetry plane XX′. Each of the gates TG1, TG2 forms in the substrateregion PPW a channel region extending between the gate and the oppositewell PNW1, PNW2, and between the well and the region SNN1, SNN2. Thus,the gates TG1, TG2 are capable of efficiently draining the charges CHaccumulated in the wells PNW1, PNW2, both deep down and on the surfaceof the wells, to send them to the read region SNN.

Thus, the substrate region PPW delimited on three sides by threeadjacent trenches DTI and on one side by one of the gates GT1, GT2,forms a photodiode region or an electric charge collecting region.

As an example, the isolating trenches DTI and the gates TG1, TG2 canextend over a depth of 1.5 to 3.5 μm, and have a width of 0.1 to 0.3 μm,for example 0.15 μm. The isolating layer TGI1, TGI2 in the trenchesforming the gates TG1, TG2 can have a thickness of 10 to 20 nm. Thewidth of a pixel (distance between two consecutive parallel trenchesDTI) may be lower than 2 μm, for example reduced to 1.1 μm.

The pixel structure that has just been described enables two photodiodesof the size of the one represented on FIGS. 1 and 2, and two transfergates to be formed in a single pixel, without increasing the surfacearea occupied by the pixel, while increasing the channel surface area ofthe transfer transistors. Furthermore, the transfer gates TG1, TG2substantially extend over the entire height of the wells PNW1, PNW2,which enables the electric charges accumulated in the wells to beefficiently brought up to the gate contact TG on the front face of thesubstrate.

FIGS. 8A to 8H represent an SOI-type substrate at different steps ofmanufacturing the pixel PXL. On FIG. 8A, the substrate comprises thesemiconductor substrate region PPW, for example made of silicon,comprising a front face on which are deposited a layer IL1 made of anisolating material such as silicon oxide, and on the layer IL1, a layerSL made of a semi-conductive material, for example silicon formed byepitaxy.

FIG. 8B shows steps of forming deep trenches and of filling thesetrenches with an isolating material such as silicon oxide, to form theisolating trenches DTI and the trenches GT1, GT2 containing theisolating material of the vertical gates TGI1, TGI2. FIG. 8B also showsa step of forming the wells PNW1, PNW2 between the trenches DTI and thegate trenches, by implanting dopants deep in the substrate region PPW.

FIG. 8C shows a step of forming a hole H1 between the trenches GT1, GT2filled with isolating material. The hole H1 is produced through thelayers SL and IL1, sufficiently deep to reach the substrate region PPW,for example at a level corresponding to the level of the top part (frontface of the substrate) of the wells PNW1, PNW2 in the substrate regionPPW. During this step, another hole of the same depth as the hole H1 canbe formed for the substrate bias region BBP.

FIG. 8D shows steps of forming the conduction region SNN of the readnode SN, by filling the hole H1 with a semi-conductive material, forexample silicon deposited by epitaxy. To form the substrate bias regionBBP, the hole can also be filled in the same way when filling the holeH1. The region SNN is then formed by implanting dopants in the materialfilling the hole H1, so as to form the area SNN1 on the surface, moreheavily doped than the area SNN2 deep down. The substrate bias regionBBP is also formed by implanting appropriate dopants in the materialfilling the hole.

FIG. 8D also shows a step of forming trenches GT3, GT4 in the isolatingmaterial TGI1, TGI2 filling the trenches GT1, GT2. The trenches GT3, GT4are formed so as to leave a thin isolating layer GTI1, GTI2 on the wallsand the bottom of the trenches GT1, GT2.

FIG. 8E shows a step of partially filling the trenches GT3, GT4 formedin the isolating material TGI1, TGI2 with a conductive material such aspolycrystalline silicon. Thus, the trenches GT3, GT4 are filled up to alevel lower than the isolating layer IL1. FIG. 8F shows a step offilling the trenches GT3, GT4 with an isolating material TGI3, such assilicon oxide, up to the front face of the external layer SL.

FIG. 8G shows a step of forming holes H2 in the isolating layers TGI3filling the top part (front face of the substrate) of the gate trenchesGT1, GT2, along the trenches DTI. FIG. 8H shows a step of filling theholes H2 with the conductive material forming the gates GT1, GT2, so asto form the columns TGC1, TGC2.

It shall be noted that the columns TGC1, TGC2 can be formed by fillingthe trenches GT3, GT4 with conductive material up to the front face ofthe layer SL, by etching the conductive material in the trenches GT3,GT4 up to a level lower than the isolating layer IL1, by leaving thecolumns TGC1, TGC2, and by filling the holes thus formed with anisolating material TGI3.

FIGS. 9 and 10 represent a pixel structure PXL1 according to anotherembodiment. FIGS. 9 and 10 correspond to views similar to the views ofFIGS. 5 and 7. The pixel structure PXL1 differs from the pixel structurePXL in that the wells PNW1, PNW2 are linked to each other by the partsof their faces opposite the side not covered with a gate GT1, GT2, so asto form a single well PNW. Thus, the well PNW has a U-shaped section ina plane parallel to the front face of the substrate region PPW. The wellPNW extends along three sides of the pixel PXL1 delimited by a firstpair of parallel trenches DTI and a second pair of trenchesperpendicular to the trenches of the first pair. Inside the U shape ofthe well PNW, the read region SNN, and the gates GT1, GT2 are formed,and between the gates, a substrate bias region BBP1 is formed. Like thepixel structure PXL, the regions SNN and BBP1 reach the substrate regionPPW beneath the isolating layer IL1. The reset transistor T3 is thusrotated by 90° around the region SNN, in relation to the pixel structurePXL. Thus, the transistor T3 comprises a conduction region formed by theregion SNN of the transistor T3, a conduction region RSN1 formed abovethe well PNW, and a gate stack RSG1 above a channel region formedbetween the regions SNN and RSN1.

FIGS. 11 and 12 represent a pixel structure PXL2 according to anotherembodiment. FIGS. 11 and 12 correspond to views similar to the views ofFIGS. 5 and 7. The pixel structure PXL2 differs from the pixel structurePXL1 in that the two embedded vertical gates TG1, TG2 are linked to eachother by a vertical gate portion bypassing the read region SNN, so as toform a single gate TTG having a U-shaped section in a plane parallel tothe front face of the substrate PPW. A single or both columns TGC1, TGC2can link the gate TTG to the front face of the layer IL2.

In the example in FIGS. 10 and 12, the photodiode region formed in thesubstrate region PPW also has a symmetrical structure in relation to theplane XX′.

It will be understood by those skilled in the art that the presentdisclosure is susceptible of various alternative embodiments and variousapplications. In particular, the disclosure is not limited to a pixelstructure with two photodiodes or a single photodiode comprising aU-shaped electric charge collecting region. To obtain a more compactpixel structure, provision may be made to form only one parallelepipedalwell in a pixel. Thus, FIG. 13 represents a pixel structure whichdiffers from the pixel structure represented in FIG. 5 in that itcomprises a single well PNW3, and a single deep vertical transfer gateTG1. The well PNW3 extends deep in a substrate region PPW3, and overmost of the thickness (between 75 and 95%) and the width (between 85 and95%) of the latter between two trenches DTI. The well PNW3 has a widthlower than or in the vicinity of half the distance between two trenchesDTI. Furthermore, the reset transistor T3 and the source followertransistor T4 are not necessarily formed in the pixel structure, but canbe formed outside the substrate region isolated by the trenches DTI, inwhich the pixel is formed.

Furthermore, the photodiode structure associated with a deep verticaltransfer gate as described above, may be used alone in many applicationsimplementing one or several photodiodes. In this case, the transistorsT3 and T4 can also be formed elsewhere than above the electric chargecollecting region of the photodiode.

It shall also be noted that the pixel structures previously describedcan be produced in a conventional semiconductor substrate rather than inan SOI-type substrate. In this case, the reset transistors T3 and sourcefollower transistors T4 are formed so as to be isolated by an isolatinglayer from the substrate part in which the photodiode is formed.

It will be understood by those skilled in the art that the features ofthe various embodiments described above can be combined in various ways.

The various embodiments described above can be combined to providefurther embodiments. These and other changes can be made to theembodiments in light of the above-detailed description. In general, inthe following claims, the terms used should not be construed to limitthe claims to the specific embodiments disclosed in the specificationand the claims, but should be construed to include all possibleembodiments along with the full scope of equivalents to which suchclaims are entitled. Accordingly, the claims are not limited by thedisclosure.

1. A device, comprising: a substrate that includes a P-conductivity typesubstrate region, the P-conductivity type substrate region having afirst surface that is opposite to a second surface; a first electriccharge collecting region and a second electric charge collecting region,each configured to collect electric charges when the second surface ofthe substrate region receives light, the first and second collectingregions each including an N-conductivity type region formed in thesubstrate region, each N-conductivity type region being spaced from thefirst surface of the substrate region and spaced from the second surfaceof the substrate region; an N-conductivity type read region, formed inthe substrate region and positioned between the first and secondcollecting region; a deep isolating trench positioned between theN-conductivity type regions of the first and second collecting regions;and a first isolated transfer gate and a second isolated transfer gate,disposed in the substrate region between the first and second collectingregions and next to the read region, and the first and second transfergates being configured to receive a gate voltage to transfer electriccharges stored in the first and second collecting regions, respectivelytoward the read region, the first and second transfer gates being formedin a deep isolating trench.
 2. The device according to claim 1,comprising first and second conductive column coupled to the first andsecond transfer gate, respectively, the substrate including an isolatinglayer on the first surface of the P-conductivity type substrate region,the first and second conductive columns extending through the isolatinglayer to a front face of the substrate.
 3. The device according to claim1 wherein each transfer gate has a lateral surface corresponding to anddisposed opposite a right or left half of a lateral face of one of theN-conductivity type regions.
 4. The device according to claim 1 whereinthe first transfer gate extends in the substrate region over a depth of1.5 to 3.5 μm, and over a width of 0.1 to 0.3 μm.
 5. The deviceaccording to claim 1 wherein the first transfer gate is isolated fromthe substrate region by an isolating layer that has a thickness between10 and 20 nm.
 6. The device according to claim 1 wherein the first andsecond collecting regions have a symmetrical structure including the twoN-conductivity type regions and the first and second transfer gates eachdisposed between one of the two N-conductivity type regions and the readregion, to transfer electric charges stored in the collecting regionstoward the read region.
 7. A device, comprising: a substrate thatincludes a P-conductivity type substrate region, the P-conductivity typesubstrate region having a first surface that is opposite to a secondsurface; a U-shaped electric charge collecting region configured tocollect electric charges when the second surface of the substrate regionreceives light, the collecting region including a U-shapedN-conductivity type region formed in the substrate region in a planeparallel to the first surface of the substrate region; an N-conductivitytype read region, formed in the substrate region and positioned betweenbranches of the U-shaped collecting region; a deep isolating trenchpositioned between the branches of the collecting region; and anisolated transfer gate disposed in the substrate region between thebranches of the collecting region and next to the read region, andconfigured to receive a gate voltage to transfer electric charges storedin the collecting regions toward the read region, the transfer gatebeing formed in deep isolating trench.
 8. The device according to claim7 wherein the collecting region has a symmetrical structure and thetransfer gate has a symmetrical structure.
 9. The device of claim 7wherein the collecting region is spaced from the first surface of thesubstrate region and spaced from the second surface of the substrateregion to be positioned within the substrate region.
 10. The deviceaccording to claim 7 wherein the transfer gate is positioned between thebranches of the U-shape of the N-conductivity type region and ispositioned around the read region.
 11. An image sensor, comprising: asubstrate that includes a P-conductivity type substrate region, theP-conductivity type substrate region having a first surface that isopposite to a second surface; a plurality of pixels formed in thesubstrate, each pixel including a first photodiode and a secondphotodiode, the first and second photodiodes including: a first electriccharge collecting region and a second electric charge collecting region,each configured to collect electric charges when the second surface ofthe substrate region receives light, the first and second collectingregions each including an N-conductivity type region formed in thesubstrate region, each N-conductivity type region being spaced from thefirst surface of the substrate region and spaced from the second surfaceof the substrate region; an N-conductivity type read region, formed inthe substrate region and positioned between the first and secondcollecting region; a deep isolating trench positioned between theN-conductivity type regions of the first and second collecting regions;and a first isolated transfer gate and a second isolated transfer gate,disposed in the substrate region between the first and second collectingregions and next to the read region, and the first and second transfergates being configured to receive a gate voltage to transfer electriccharges stored in the first and second collecting regions, respectivelytoward the read region, the first and second transfer gates being formedin a deep isolating trench.
 12. The sensor according to claim 11 whereinthe read region forms a conduction region of a reset transistorconfigured to set a voltage of the read region.
 13. The sensor accordingto claim 11 wherein each pixel includes a source follower transistorhaving a gate terminal coupled to the read region, the source followertransistor being formed above the N-conductivity type region, anisolating layer being arranged between the source follower transistorand the N-conductivity type region.
 14. The sensor of claim 11 whereinthe pixels are isolated from each other by deep isolating trenches orembedded vertical gates.
 15. A method, comprising: manufacturing animage sensor having a plurality of pixels, the manufacturing including:forming in a P-conductivity type substrate region a first and secondelectric charge collecting region for collecting electric charges thatappear when a rear face of the substrate region receives light; formingthe first and second collecting regions to each have an N-conductivitytype region in the substrate region; forming in the substrate region anN-conductivity type read region; forming in the substrate region a deepisolating trench extending between the first and second collectingregions and next to the read region; and forming an isolated transfergate in the isolating trench, the transfer gate being configured forreceiving a gate voltage to transfer electric charges stored in thecollecting region toward the read region.
 16. The method according toclaim 15 wherein the substrate is of silicon-on-insulator type.
 17. Themethod according to claim 16, further comprising forming the read regionand a substrate bias region by epitaxy of a semi-conductive material ina hole passing through an isolating layer of the substrate to reach thesubstrate region.